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Chapter 16

Chapter 16. Section 4: Water in the Atmosphere. Section 4. Humidity (is a measure of the amount of water vapor in the air) The movement of water between the atmosphere and Earth’s surface is called the water cycle

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Chapter 16

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  1. Chapter 16 Section 4: Water in the Atmosphere

  2. Section 4 • Humidity (is a measure of the amount of water vapor in the air) • The movement of water between the atmosphere and Earth’s surface is called the water cycle • Evaporation is the process by which water molecules in liquid water escape into the air as water vapor • Warm air can hold more water vapor than cool air

  3. Section 4 • Humidity • Relative Humidity (is the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold at a particular temperature) • Air with a relative humidity of 100% is said to be saturated • If the air had 4 grams of water vapor, the relative humidity would be half, or 50%

  4. Section 4 • Humidity • Measuring Relative Humidity • Relative humidity can be measured with an instrument called a psychrometer (has two thermometers, a wet-bulb thermometer and a dry-bulb thermometer) • The bulb of the wet-bulb thermometer has a cloth covering that is moistened with water; when the psychrometer is “slung” or spun by its handle, air blows over both thermometers • Cause the wet-bulb thermometer is cooled by evaporation, its reading drops below that of the dry-bulb thermometer

  5. Section 4 • Measuring Relative Humidity • If the relative humidity is high, the water on the wet bulb evaporates slowly, and the wet-bulb temperature does not change • If the relative humidity is low, the water on the wet bulb evaporates rapidly, and the wet-bulb temperature drops • The relative humidity can be found by comparing the temperatures of the wet-bulb and dry-bulb thermometers

  6. Section 4 • How Clouds Form • Clouds form when water vapor in the air condenses to form liquid water or ice crystals • Molecules of water vapor in the air become liquid water in the process of condensation • Two conditions are required for condensation • Cooling of the air • Presence of particles in the air

  7. Section 4 • How Clouds Form • The Role of Cooling • Cold air holds less water vapor than warm air • As air cools, the amount of water vapor it can hold decreases • The water vapor condenses into tiny droplets of water or ice crystals • The temperature at which condensation begins is called the dew point • If the dew point is above freezing, the water vapor forms water droplets • If the dew points is below freezing, the water vapor may change directly into ice crystals

  8. Section 4 • How Clouds Form • The Role of Particles • For water vapor to condense, tiny particles must be present so the water has a surface on which to condense • In cloud formation, most of these particles are salt crystals, dust from soil, and smoke • Liquid water that condenses from the air into a cooler surface is called dew • Ice that has been deposited on a surface that is below freezing is called frost

  9. Section 4 • Types of Clouds • Scientists classify clouds into three main types based on their shape: cirrus, cumulus, and stratus • Clouds are further classified by their altitude • Cirrus Clouds (wispy, & feathery) • Form only at high levels, where temperatures are very low, made of ice crystals • Cirrocumulus clouds indicate that a storm is on its way

  10. Section 4 • Types of Clouds • Cumulus Clouds (fluffy, rounded piles of cotton) • These clouds are not very tall usually indicate fair weather • Common on sunny days • Cumulonimbus clouds, often produce thunderstorms • Stratus Clouds (form flat layers) • Usually cover all or most of the sky and are a uniform dull, gray color • As these clouds thicken, they may produce drizzle, rain, or snow (nimbostratus clouds)

  11. Section 4 • Types of Clouds • Altocumulus & Altostratus • These are “middle level” clouds that are higher than regular cumulus and stratus clouds, but lower than cirrus and other “high” clouds • Fog(clouds that form at or near the ground) • Often forms when the ground cools at night after a warm, humid day • The ground cools the air just above the ground to the air’s dew point • The next day the heat of the morning sun “burns” the fog off as its water droplets evaporate • More common by bodies of water or low-lying marshy areas

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